SKELETAL MUSCULATURE OF THE KING SALMON. 49 



ments. If, during this general propulsive motion, the form and shape of the caudal 

 fin is adjusted as it would be by graduated contractions of the deep caudal muscles, it 

 is obvious that the fin will be the guiding rudder controlling the exact direction of the 

 forward movement. In closing the discussion of this phase of our subject we may 

 reiterate once more the statement previously made, that these deep caudal muscles 

 control the positions of the caudal fin which will adapt it to the purposes of a rudder. 

 The great lateral muscles furnish the power which acts on the caudal fin as a whole, 

 furnishing a piscine propeller seldom equaled and never excelled in the aquatic world. 



MUSCLES OF THE PECTORAL GIRDLE. 



The pectoral muscles of the European salmon, Salmo salar, have been briefly 

 described by Harrison," and more recently described and figured by Pychlau.*" The 

 development is given by Harrison, and by Vogel '^ for the trout, Trulta fario. Many 

 instructive comparative points in the myology of fishes are to be had from the exhaustive 

 papers by Allis ^ on Aniia calva and Scrombcr scromhcr. 



In Oncorhynckus tschawylscha the pectoral fin has 14 rays, the basal or external 

 one being markedly heavier and the others successively more slender. The base of 

 each half ray is cur\'ed sharply toward the median border of the fin. The two halves of 

 each ray are widely separated at the base. The series of rays is seated like a saddle 

 across the skeletal ridge of the basalia, forming a very mobile joint, as described by 

 Pychlau for Salmo salar. This type of joint is also found in all the salmon fins, but 

 with modifications. 



ABDUCTOR PECTORALIS SUPERFICIAUS. 



This muscle arises from the anterior ventral border and the inner or median ventral 

 margin of the coracoid as far back as the base of the fin. Its surface of origin along the 

 coracoid is widest about one-third the distance from the anterior end of the coracoid, 

 where it covers a surface of about 9 mm. wide in a standard fish. The median line of 

 origin is along the ventral ridge on the coracoid, covering this ridge for one-third its 

 length. The fibers of the muscles run back over the deep abductor to a tendinous 

 insertion in the tips of the processes of the ventral half rays. The ventral surface of the 

 muscle near its origin has its tendon joined by the fibers of the protractor ischii. These 

 occasionally spread fan-shaped over the surface of the angle between the ventral ends 

 of the clavicles. The external fibers of the pectoralis superficialis are in close approxi- 

 mation to, and have tendons intimately fused with, the internal portion of the profundus. 



The action of the superficialis is to bend the fin downward and forward and to 

 close the rays. 



" Harrison, Ross G. ; Die Entwicklung der unpaaren und paarigen Flossen der Teleostier. Archiv fiir Mikroskopische 

 Anatomic, bd. 46, 1S95, p. 500-578. 



^ Pychlau, Waldemar: Untersuchungen an den Brustflossen einiger Teleostier. JenaischeZeitschrift, bd. 43, 190S, p. 692-728. 



c Vogel, Richard; Die Entwickelung des Schultergiirtels und des Brustflossenskelettes der Forelle (Trutta fario). Jenaische 

 Zeitschrift, bd. 45, 1909, p. 499-544. 



(^ Allis, Edward Phelps: The skull and the cranial and first spinal muscles and nerves in Scomber scomber. Journal of Mor- 

 phology*. 1903. vol. iS, 1903. p. 45-328. 



Same author: The cranial muscles and cranial and first spinal nerves in Amia calva. Journal of Morphology, vol. 12, 1R97, 

 p. 4S7-80S. 



